A position-measuring device of this kind is described, for example, in U.S. Patent Application Publication No. 2005/0023450. In addition to a measuring graduation, e.g., taking the form of a linear reflection measuring graduation, the device includes a scanning unit displaceable relative thereto in at least one measuring direction. Provided on the side of the scanning unit are a light source as well as a detector arrangement in the form of a periodic detector array.
In the event of the relative movement of the scanning unit and measuring graduation, there results in the detection plane a fringe pattern, modulated as a function of the displacement, which is detected by the detector arrangement and converted into scanning signals able to be further processed. In this context, because the detector arrangement is arranged in the form of a periodic detector array, a plurality of out-of-phase scanning signals are generated in a conventional manner.
In such systems, generally, the aim is for the light source used and the detector arrangement used to be arranged to the greatest extent possible in the same plane. This may be achieved, for example, by disposing the light source in a central cavity in a carrier substrate, the cavity being surrounded by the detector elements of the detector array. However, this is associated with significant manufacturing expenditure. Thus, the carrier substrate must be formed with a suitable cavity. In addition, the contacting of the light source in the cavity is relatively difficult.
With regard to such systems, U.S. Patent Publication No. 2004/0155178 describes in FIGS. 13 and 14 that by arranging transmissive optical elements between the light source and measuring graduation, it is possible to precisely adjust the geometric-spatial position of a virtual point light source. However, in so doing, the transmissive optical elements displace the position of the virtual point light source markedly before the detection plane. That is to say, the above-mentioned requirement with respect to the arrangement of the light source in the plane of the detector arrangement cannot be satisfied by the measures proposed. A change, dependent on the scanning distance, in the periodicity of the fringe pattern generated in the detection plane results as an unwanted consequence. However, the aim is for a constant fringe-pattern period in the detection plane, even if the scanning distance possibly fluctuates.